1. Field of the Invention
The present invention relates to an exhaust gas purification device for an internal combustion engine. More specifically, the invention relates to an exhaust gas purification device equipped with an NOx occluding and reducing catalyst which absorbs NOx in the exhaust gas when the air-fuel ratio of the exhaust gas flowing in is lean and releases the absorbed NOx when the oxygen concentration in the exhaust gas flowing in has decreased.
2. Description of the Related Art
There has been known an NOx occluding and reducing catalyst which absorbs NOx (nitrogen oxides) in the exhaust gas when the air-fuel ratio of the exhaust gas flowing is lean and releases the absorbed NOx when the oxygen concentration in the exhaust gas flowing in has decreased.
An exhaust gas purification device using the NOx occluding and reducing catalyst of this type has been disclosed in, for example, Japanese Patent No. 2600492. According to this exhaust gas purification device, the NOx occluding and reducing catalyst is disposed in an exhaust gas passage of an engine which operates at a lean air-fuel ratio to absorb NOx in the exhaust gas when the engine is operating at a lean air-fuel ratio and, when the amount of NOx absorbed by the NOx occluding and reducing catalyst has increased, a rich-spike operation is executed to operate the engine at an air-fuel ratio smaller than the stoichiometric air-fuel ratio (i.e., a rich air-fuel ratio) for a short period of time, thereby to release the absorbed NOx from the NOx occluding and reducing catalyst and to purify the released NOx by the reduction. That is, as the air-fuel ratio of the exhaust gas changes to a rich air-fuel ratio, the oxygen concentration in the exhaust gas sharply decreases compared to that of the exhaust gas of an air-fuel ratio larger than the stoichiometric air-fuel ratio (i.e., lean air-fuel ratio), and the amounts of unburned HC and CO components sharply increase in the exhaust gas. Therefore, when the engine operating air-fuel ratio is changed over to a rich air-fuel ratio due to the rich-spike operation, the air-fuel ratio of the exhaust gas flowing into the NOx occluding and reducing catalyst changes from a lean air-fuel ratio to a rich air-fuel ratio, whereby NOx is released from the NOx occluding and reducing catalyst due to a decrease in the oxygen concentration in the exhaust gas. The exhaust gas having a rich air-fuel ratio contain unburned HC and CO components in relatively large amounts. Therefore, NOx released from the NOx occluding and reducing catalyst is reduced by reacting with the unburned HC and CO components in the exhaust gas.
According to the exhaust gas purification device disclosed in the above-mentioned Japanese Patent No. 2600492, NOx emitted while the engine is operating at a lean air-fuel ratio are absorbed by the NOx occluding and reducing catalyst, and the rich-spike operation is effected to release NOx from the NOx occluding and reducing catalyst and, at the same time, to purify NOx by the reduction.
However, it has been found that when NOx are released from the NOx occluding and reducing catalyst and are purified by the reduction by executing the rich-spike operation, unpurified NOx are often released, without being reduced, from the NOx occluding and reducing catalyst at the beginning of the rich-spike operation.
Though it has not yet been fully clarified why unpurified NOx are released from the NOx occluding and reducing catalyst at the beginning of the rich-spike operation, the cause is attributed to the fact that the NOx occluding capacity (maximum NOx occluding amount) of the NOx occluding and reducing catalyst changes in accordance with the air-fuel ratio.
When the air-fuel ratio is sharply changed to a rich air-fuel ratio by the rich-spike operation while the engine is operating at a very lean air-fuel ratio, a torque shock due to a sudden change in the engine output torque occurs. In the actual operation of the engine, therefore, when the rich-spike operation is executed while the engine is operating at a very lean air-fuel ratio (e.g., an air-fuel ratio of about 30), the air-fuel ratio of the engine is not immediately changed to a rich air-fuel ratio in order to suppress a sudden change in the engine output torque. Namely, when the rich-spike operation is performed, the engine operating air-fuel ratio is first changed to a lean air-fuel ratio relatively near a stoichiometric air-fuel ratio (a moderate lean air-fuel ratio of about 20) from a very lean air-fuel ratio (about 30) and, after operating the engine for a period of several revolutions of the engine at this moderate lean air-fuel ratio, the engine operating air-fuel ratio is changed to a rich air-fuel ratio.
By operating the engine in a moderate lean air-fuel ratio region before changing to a rich air-fuel ratio, the operating air fuel ratio of the engine gradually changes from a very lean air-fuel ratio to a rich air-fuel ratio and, thereby, a sudden change in the operating air-fuel ratio and a resulting torque shock do not occur. When the rich spike operation is executed, therefore, the engine is operated in a region of a moderate lean air-fuel ratio for some time.
However, it has been found that the NOx occluding capacity of the NOx occluding and reducing catalyst is affected by the air-fuel ratio of the exhaust gas that is flowing in and, in the moderate lean air-fuel ratio region, decreases as the air-fuel ratio becomes low. FIG. 11 is a graph illustrating a relationship between the NOx occluding capacity (maximum NOx occluding amount) of the NOx occluding and reducing catalyst and the air-fuel ratio of the exhaust gas flowing in. As shown in FIG. 11, the NOx occluding capacity of the NOx occluding and reducing catalyst remains nearly constant irrespective of the air-fuel ratio in a region where the air-fuel ratio is larger than 20. In a region where the air-fuel ratio is smaller than 20, however, the NOx occluding capacity decreases with a decrease in the air-fuel ratio of the exhaust gas (decreases as the air-fuel ratio approaches the stoichiometric air-fuel ratio), and becomes 0 at the stoichiometric air-fuel ratio.
Therefore, when the NOx occluding and reducing catalyst enters into a region of moderate lean air-fuel ratios in which the air-fuel ratio is smaller than 20 from the lean air-fuel ratio region where the air-fuel ratio is larger than 20 where NOx is absorbed nearly up to its maximum NOx occluding amount, the whole amount of occluded NOx is no longer held due to the decrease in the occluding capacity; i.e., NOx is released by an amount corresponding to a difference between the amount of NOx actually occluded and the maximum occluding amount (an amount corresponding to hatched area in FIG. 11). Besides, in the region of moderate lean air-fuel ratios, since HC and CO components are contained in very small amounts in the exhaust gas, NOx that is released is not reduced on the NOx occluding and reducing catalyst and is released from the NOx occluding and reducing catalyst without being purified.
In the foregoing was described the case where the engine operating air-fuel ratio has changed from a lean air-fuel ratio larger than 20 over to the region of moderate lean air-fuel ratios due to the rich-spike operation. The same problem, however, could arise when the engine operating air-fuel ratio changes from a rich air-fuel ratio to the region of moderately lean air-fuel ratios.
It has been known that the amount of NOx emitted from the engine increases, for example, when the engine operating air-fuel ratio falls in the region of moderate lean air-fuel ratios. FIG. 12 is a diagram illustrating a relationship between the operating air-fuel ratio (combustion air-fuel ratio in the combustion chamber of an engine) of a lean-burn engine that operates at a lean air-fuel ratio and the NOx concentration in the exhaust gas from the engine. As represented by a curve A in FIG. 12, the amount of NOx emitted by the engine increases with an increase in the operating air-fuel ratio near the stoichiometric air-fuel ratio, becomes a maximum in a region where the air-fuel ratio is from 20 to 23 and, then, decreases with a further increase in the air-fuel ratio. In an engine having an exhaust gas purifying catalyst such as a three-way catalyst disposed in an exhaust gas passage upstream of the NOx occluding and reducing catalyst, NOx in the exhaust gas are nearly completely reduced at an air-fuel ratio smaller (more rich) than the stoichiometric air-fuel ratio. In this case, as represented by a curve B in FIG. 12, the concentration of NOx in the exhaust gas flowing into the NOx occluding and reducing catalyst on the downstream side of the exhaust gas purifying catalyst becomes nearly zero when the air-fuel ratio is smaller than the stoichiometric air-fuel ratio, and sharply rises near the stoichiometric air-fuel ratio to come into agreement with the curve A.
Therefore, when the engine is operated in the region of moderate lean air-fuel ratios (region from the stoichiometric air-fuel ratio up to an air-fuel ratio of about 20), the amount of NOx in the exhaust gas flowing from the engine and into the NOx occluding and reducing catalyst increases up to nearly a maximum amount. As mentioned above, on the other hand, the NOx occluding capacity of the NOx occluding and reducing catalyst decreases in the region of moderate lean air-fuel ratios. In this region, therefore, it could happen that NOx in the exhaust gas are not all absorbed when the amount of NOx emitted by the engine increases during the air-fuel ratio is passing through the region of moderate lean air-fuel ratios, and NOx in the exhaust gas are released without being purified from the NOx occluding and reducing catalyst even if the amount of NOx occluded in the NOx occluding and reducing catalyst is relatively small and the spontaneous release of NOx from the NOx occluding and reducing catalyst does not occur.
In the practical operation, furthermore, the engine operating air-fuel ratio may be changed over a wide range of from a rich air-fuel ratio to a lean air-fuel ratio depending upon the operating conditions (load, etc.) of the engine, and unpurified NOx are often released from the NOx occluding and reducing catalyst when the operating conditions are changed and the engine is operated in the region of moderate lean air-fuel ratios in addition to the case where the rich-spike operation is executed. That is, unpurified NOx are often released from the NOx occluding and reducing catalyst when the engine operating air-fuel ratio has changed within a range of lean air-fuel ratios in addition to when the rich-spike operation is executed (in this specification, the release of NOx from the NOx occluding and reducing catalyst due to a change in the air-fuel ratio within the range of the moderate lean air-fuel ratios is called "spontaneous release", to make a distinction between it and the intended release of NOx from the NOx occluding and reducing catalyst by executing the rich-spike operation).
In general, the lean-burn engine is in many cases operated in a region of lean air-fuel ratios of larger than 20. In an engine for a vehicle, when a large engine output is required such as at the time of acceleration or climbing a hill, or when a negative pressure is required for applying the brake, the operation air-fuel ratio is often changed from a lean air-fuel ratio to a rich air-fuel ratio. In such a case, the engine operating air-fuel ratio is changed from a lean air-fuel ratio to a rich air-fuel ratio passing through an intermediate region of moderate lean air-fuel ratios in order to avoid a sharp change in the output torque caused by a sharp change in the air-fuel ratio like when the rich-spike operation is executed. At the time of acceleration or climbing a hill, therefore, the air-fuel ratio of the exhaust gas flowing into the NOx occluding and reducing catalyst passes through the region of moderate lean air-fuel ratios before it is changed over to the rich air-fuel ratio and, hence, NOx are often spontaneously released from the NOx occluding and reducing catalyst.
Thus, release of unpurified NOx from the NOx occluding and reducing catalyst for every rich-spike operation of the engine or for every change in the engine operating air-fuel ratio due to a change in the operating conditions, causes a problem of a decrease in the NOx purification efficiency as a whole.